Host-symbiont complementarity

Question 1

Simple, tight and ancient mutualism

Answer 1

Buchnera provides essential amino
acids

Aphids provide nutrients, housing and
transmission between generations

Question 2

Diverse Modes of Transmission & Location

Answer 2

horizontal - extracellular (bobtail squid)
vertical - extra (earthworm)/intracellular (weevil)

Question 3

vertically-transmitted intracellular symbionts have

Answer 3

entire life cycle within host
co-speciation

Question 4

Effect on symbiont genome evolution

Answer 4

increasing genome degradation
- genome reduction
mutation bias towards AT

Question 5

Drivers of genome degradation

Answer 5

“mullers ratchet”:
genetic drift
-Recurrent bottlenecks during
transmission
-small effective population size
-lack of recombination
-fixation of slightly deleterious mutations
-pseudogeniziation and gene loss

Question 5

Stable host environment →

Answer 5

→ diminishes need for many genes

Question 6

Metabolic complementarity from host or co-symbionts →

Answer 6

diminishes need for many genes

Question 7

lower limit for genome size?

Answer 7

Theoretical lower limit: 80-100 kb

Question 8

symbiont vs organelle

Answer 8

Evolution of organelles:
Transfer of bacterial genes to host nucleus, and of host synthesise most of the proteins

Intracellular symbionts:
All functional proteins are synthesized by the symbiont itself

Question 9

Accumulation of mobile genetic elements is a

Answer 9

hallmark for early genome evolution in symbiosis

Question 10

Symbionts appear to have higher density of

Answer 10

mobile elements than free-living relatives
(but degree is variable)

Question 11

paradox of the earthworm symbiosis

Answer 11

Vertical transmission
with a twist:
”free-living” stage in
the cocoon
prevent
genome degradation